Stefano Tirelli

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Stress-strain behaviour of MoRuB glassy metals
ChenYang Wang Graduate Student Caltech Now at
University of Stanford

• Stefano Tirelli
• Undergraduate student
• University of Pisa
• Italy

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Physical properties studied

• Youngs Modulus
• Mechanical hysteresis
• Yield Point
• High yield point allows THINNER suspensions and
  less energy dissipation
• Structural modifications
• Shear bands
• Crack propagation (upcoming)

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How to study stress/strain of MoRuB?
Load frame, operational setup
Load frame and cell courtesy of Robert Rogan,
Materials Science
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How to measure stress?
Micro Load Cell, maximum load 1000lbs
(oversized!) Wheatstone-bridge based Present
resolution 50 grams
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How to measure strain?
LVDT Linear Variable Differential Transformer
Custom designed micro-LVDT and holders
Current flow
Differential measurement high resolution,
rejection of noise
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Linear response
Linear regime
Voltage (Volts)
1 Volt
1mm
Displacement (mm)
Present resolution 15nm
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The stress/strain chart
The stress\strain chart give us a clear picture
of many properties of the material
Yield point
Slope Youngs modulus
Breaking point
Ductile deformation (not present in Glassy
metals)
Plastic deformation (not present in Glassy
metals)
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Stress/strain chart for MoRuB
First cycles low load
Non-rigidity of the system
Hysteresis NOT due to sample
Load (Kg)
Sample tested Mo50.4Ru33.6B16 Cross
section 3.3mm x 50um
Stress load g 1/cross-section
Displacement (microns)
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Stress/strain curves for MoRuB
Crack formation
Material hysteresis
Load (Kg)
Permanent deformation No breakdown failure as in
crystals!
Displacement (microns)
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Why low values for yield point?
Youngs modulus Boron 16 174 GPa
Yield point (lower limit!) Boron 16 1.34
Gpa (upper limit 5.2GPa)
Error 15 mainly due to poor thickness Measureme
nt. Solution precision-micrometer Upcoming.
Non-uniformity of stress effective cross-section
is LOWER than the measured one. Solution
self-aligning swivel holders (already in
production)
Rotating swivel
Two LVDTs for detecting torsion!
LVDT
Sample holder
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Why low values for yield point?
Nucleation of cracks. To take good measurements
we need regular borders without weak point for
crack nucleation
EDM Cut Local melting Possible formation Of
crystals on edges
Scissor cut Very irregular and unreliable!
Electropolished cut The best!
Nucleation of cracks causes premature failure of
the material!
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Structural effects of stress shear bands
Shear bands
Details of a MoRuB broken sample
Who knows about shear bands? I dont, but theyre
nice.
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What needs to be done

• Testing on electropolished samples to obtain a
  value close to the one calculated by Vicker
  hardness test (5.2GPa).
• Study of crack propagation.
• Poissons modulus.
• Observation of shear bands during formations
  load frame is designed to fit into an SEM casing.

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Thanks to
Riccardo DeSalvo - Mentor Prof. Francesco
Fidecaro - University of Pisa ChenYang Wang -
Graduate Student, LabMate Hareem Tariq -
Graduate Student Prof. William Johnson Robert
Rogan - Materials Science Michael Hall Brian
Emmerson Eric Kort Maddalena Mantovani Barbara
Simoni